Variation was studied at 13 electrophoretically detected allozyme loci for samples of six populations of the polytypic snail Neotricula aperta; this was the first time that topotypic material from southern Laos and material from Kampuchea had been so examined. In addition, samples of all three strains of N. aperta from North East Thailand were examined and the findings compared with those of Staub et al. (1990). The samples were taken from the Mekong and Mul rivers of the Lower Mekong Basin. A small sample was also included from a γ-strain population in the Xé-Bang-Fai (XBF) river of central Laos. The γ-strain population of southern Laos and of Kampuchea has been shown to act as an intermediate host for Schistosoma mekongi.

In contrast to the work of Staub et al. (1990) significant multilocus genetic differences (Nei's D, DN = 0.6–1.2) were detected between the three strains from Thailand and no cryptic taxon, that included both α- and γ-strains could be demonstrated. However, there was agreement with the 1990 report in that the β-strain was divisible into two new taxa, with a reduction in heterozygote deficiency, although the genetic distance was not marked (DN= 0.12). Significant genetic distances were found between γ-strain snails from the type locality and β- and γ-strain samples from North East Thailand (DN = 2.0 and 1.5, respectively). On the basis of genetic distance measures, the Β-strain is probably a sibling species of N. aperta and the γ-strain of North East Thailand may also be a separate species. The population at XBF appears to be N. aperta s.s. The public health implications of the findings are discussed, particularly in the context of Mekong river water resource development.